WDM optical communication system capable of monitoring temperature

ABSTRACT

A device and method to monitor and temperature compensate change in wavelength within a WDM optical communication system is disclosed. The device includes a laser diode for generating an optical signal for temperature monitoring, a first fiber Fabry-Perot interferometer sensor unit for generating a first sensor signal according to temperature by means of the optical signal, and a processor for monitoring temperature by means of the first sensor signal and an externally provided second sensor signal and equalizing waveforms of the two sensor signals with each other. The externally provided second sensor signal is provided by a remote node that includes a second fiber Fabry-Perot interferometer sensor unit for receiving the optical signal for temperature monitoring to generate the second sensor signal according to temperature, and a waveguide grating router unit for transmitting the second sensor signal to the optical line terminal.

CLAIM OF PRIORITY

This application claims priority, pursuant to 35 U.S.C. §119, to thatpatent application entitled “WDM Optical Communication System Capable ofMonitoring Temperature,” filed in the Korean Intellectual PropertyOffice on Sep. 24, 2003 and assigned Serial No. 2003-66354, the contentsof which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wavelength division multiplexedpassive optical network, and, more particularly, to temperaturemonitoring and wavelength compensation within the WDM system.

2. Description of the Related Art

Typical optical subscriber networks use a double star structure in orderto minimize the length of an optical line needed to connect the networknodes together. From an optical line terminal (OLT) to a remote node(RN) installed at an area adjacent to the subscriber units, the OLT andRNs are connected via a single strand of optical fiber. The remotenodes, are similarly connected to each associated subscriber unitthrough a separate optical fiber.

In a wavelength division multiplexing (WDM) systems the subscriber unitsuse or are assigned different wavelengths or channels, each subscriberunit may be referred to by their associated wavelength or channelnumber. OLTs and the remote nodes must include a multiplexer unit (MUX)for multiplexing individual optical signals and a demultiplexer (DEMUX)for demultiplexing the multiplexed optical signals. A waveguide gratingrouter (WGR) is typically used as for such multiplexer/demultiplexeroperations

As the remote nodes of a WDM optical network installed at a siteadjacent to the subscriber units may be distributed over largedistances, the remote nodes may be affected by temperature change causedby changes of season or even by whether it is day and night differentlythan other RNs or the OLT. The temperature range that a remote node mustoperate over has been established by system specifications of at least120 degrees Centigrade (deg. C.), i.e., from −40 to +80 deg. C., and amaximum change rate of temperature of 1 deg. C./min.

The WGR used as a multiplexer/demultiplexer, as described above, has arate of change of wavelength with respect to temperature that isdetermined based on the materials from which the WGR is made. Forexample, when the WGR is made from general semiconductor materials, therate of change of wavelength with respect to temperature is about 0.1nm/deg. C. On the other hand, when the WGR is made from silicon dioxide(SiO₂) the rate of change of wavelength with respect to temperature isabout 0.015 nm/deg. C. See for example, ‘A Wavelength-Matching Schemefor Multiwavelength optical Links and Networks Using GratingDemultiplexers’, F. Tong, et. al, Journal of IEEE Photon. Tchnol.,Lett., seventh volume, pp. 688-690, (1995).

Consequently, the wavelength of a WDM light source for downstreamtransmission and the received wavelength of the WGR at the remote node(or the wavelength of the WGR in the OLT and the wavelength of the WGRin the remote node) may not coincide with each other, resulting in anoutput loss on channels transmitted. Crosstalk between adjacent channelsis further increased as the nominal wavelength of one channel approachesthe nominal wavelength of an adjacent channel. Hence, the transmissionperformance of a system begins to deteriorate as the temperature in theOLT and/or the remote nodes changes.

In order to prevent the transmission performance from beingdeteriorated, wavelength tracking methods have been proposed that canequalize the wavelength of the WDM light source(s) for downstreamtransmission with the wavelength of the WGR in the remote node aschanges occur according to temperature of the site. Such a wavelengthtracking methods are well-known in the art and need not be discussed indetail herein. See, for example, ‘Fiber-Grating Sensor for WavelengthTracking in Single-Fiber WDM Access PON's’, Randy Giles and Song Jiang,Journal of IEEE Photon. Tchnol., Lett., ninth volume, pp. 523-525,(1997); ‘Wavelength Tracking of a Remote WDM Router in a Passive OpticalNetwork’, D. Mayweather, et. al, Journal of IEEE Photon. Tchnol., Lett.,pp. 1238-1240 eighth volume (1996); and ‘Demonstration of a 12×155 Mb/sWDM PON Under Outside Plant Temperature Conditions’, R. Monnard, et. al,Journal of IEEE Photon. Tchnol., Lett., pp. 1655-1657, ninth volume(1997).

Conventional wavelength tracking method use a monitor channel dedicatedonly for wavelength tracking, a dedicated optical fiber for providingthe monitor channel to a central office and an optical fiber diffractiongrating. In one method, a measure of the difference between thewavelength of the WDM light source at the OLT and the wavelength of theWGR in the remote node is determined and the temperature of the WDMlight source is adjusted so that the wavelength of the light sourcecoincides with the wavelength of the WGR in the remote terminal.

Another method of equalizing the wavelength of the WDM light source fordownstream transmission in the OLT is described in Korea PatentApplication No. 1999-35226 (filed on Aug. 24, 1999, KAIST), entitled “AnApparatus and a Method for Tracking a Wavelength in a Spectrum-SlicedWDM Passive Optical Network”. The wavelength tracking method ofequalizing the wavelength of the WGR in the OLT with the wavelength ofthe WGR in the remote node proposed in Korea Patent Application No.1999-35226 is shown in FIG. 1 for the upstream transmission in a WDMpassive optical network. A similar operation is proposed for downstreamtransmission and need not be described in detail herein.

The passive optical network shown includes a central office 100, aremote node 200, and an upstream line for transmitting an upstreamsignal from remote node 200 to central office 100.

Remote node 200 includes multiplexer 201 represented by a WGR, atemperature controller 202 that provides a current to a thermoelectriccooler 220, that is used to control or adjust the temperature ofmultiplexer 201.

Central office 100 includes an erbium-doped fiber amplifier 102 foramplifying an upstream signal, demultiplexer 101, represented WGR, and awavelength tracking apparatus 110. The wavelength tracking apparatus 110adjusts a current provided in a thermoelectric cooler 120 in order tocontrol and adjust the temperature of the demultiplexer 101.

Wavelength tracking apparatus 110 includes optical coupler 104,photodetector 111, a plurality of optical couplers 105 and 106, aplurality of photodetectors 115 and 116, a plurality of diodes 117 and118, an amplifier 119, a differential amplifier 113, and a controlcircuit 112.

The optical coupler 104 is located between the erbium-doped fiberamplifier 102 in an upstream line and the demultiplexer 101 and providesoptical power passing through the upstream line to the photo detector111. In the embodiment of the invention shown the optical coupler 104distributes the received optical signal at the rate of 99:1. One skilledin the art would recognize that the coupler may distribute the opticalpower in other ratios without altering the scope of the invention. Thephotodetector 111 receives the optical signal distributed from theoptical coupler 104 and generates and outputs a voltage proportional tothe intensity of optical power.

The optical couplers 105 and 106 are connected to channels of outputterminals of the demultiplexer 101 and distribute optical poweroutputted from the demultiplexer 101 to optical power meters 107 and 108and the photodetectors 115 and 116, respectively. The photodetectors 115and 116 generate and output a voltage proportional to the intensity ofthe optical power received from the optical couplers 105 and 106.

The photodetectors 115 and 116 are connected to the amplifier 119through the diodes 117 and 118, respectively. The highest voltage of thevoltages generated by photodetectors 115 and 116 is provided to theamplifier 119 through the diode 115 or 116 and is then amplified by theamplifier 119. The differential amplifier 113 has an inverting terminalconnected to an output terminal of the amplifier 119 and a non-invertingterminal connected to an output terminal of the photodetector 111, andthe differential amplifier 113 amplifies and outputs the differencebetween two input signals. The control circuit 112 outputs a current tothe thermoelectric cooler 120 for adjusting temperature of demultiplexer101. The level of current is determined based on the size and change ofthe signal outputted from the differential amplifier 113.

In the system described above, an optical signal in each channel issplit, e.g., 10%/90%, and the intensities of the 10% signals aredetected by the photodetectors 115 and 116, and the detected intensitiesare added to each other. Then, the intensity obtained through theaddition is compared with an intensity of an optical signal beforepassing through the WGR 101. When the two intensities are different fromeach other due to abnormality of any one channel or the photodetectors,the control circuit 112 determines that a wavelength is shifted due tothe change of temperature and performs an operation such as anadjustment of temperature occurs.

Since intensities of the optical signals passing through respectivechannels must be added to each other, couplers and photodetectors mustbe provided to respective channels in order to split the opticalsignals. Therefore, the cost of a system increases. Hence, there is needfor a low cost system and method for monitoring wavelength shift andadjusting temperature to compensate for such shift.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a device in a WDMoptical communication system capable of monitoring temperature, that canprevent disagreement of pass wavelength bands by comparing outputwaveforms from fiber Fabry-Perot interferometer sensors installed atrouters at an optical line terminal and a remote node with each otherand controlling the router in the optical line terminal to have the sametemperature as that of the router in the remote node.

Another object of the present invention is to solve a problem in thatthe shift of a wavelength due to the influence of temperature occurs ina WGR that is a router of a remote node so that a pass band in the WGRdoes not coincide with that in a router of an OLT in a WDM opticalcommunication system and to reduce a crosstalk.

In order to accomplish the aforementioned object, according to oneaspect of the present, there is provided a WDM optical communicationsystem capable of monitoring temperature comprising an optical lineterminal including a laser diode for generating an optical signal fortemperature monitoring, a first fiber Fabry-Perot interferometer sensorunit for generating a first sensor signal according to a temperature bymeans of the optical signal, and a processor for monitoring temperatureby means of the first sensor signal and a second sensor signal andequalizing waveforms of the two sensor signals with each other, whereina remote node includes a second fiber Fabry-Perot interferometer sensorunit for receiving the optical signal for temperature monitoring and togenerate the second sensor signal according to a temperature, and awaveguide grating router unit for transmitting the second sensor signalto the optical line terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a construction of upstream transmission in a WDM passiveoptical network according to the prior art;

FIG. 2 shows a construction of a fiber Fabry-Perot interferometer sensorused in the present invention;

FIG. 3 shows an output waveform of a fiber Fabry-Perot interferometersensor used in the present invention; and

FIG. 4 shows a construction of a WDM passive optical network capable ofmonitoring temperature according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments according to the present invention will bedescribed with reference to the accompanying drawings. For purposes ofclarity and simplicity, a detailed description of known functions andconfiguration incorporated herein will be omitted as it may make thesubject matter of the present invention unclear.

FIG. 2 is a block diagram showing a construction of an exemplary fiberFabry-Perot interferometer sensor (FFPI) used in the present invention.

The FFPI shown includes a 2×2 coupler 21, an FFPI sensor 23, and anindex matching unit 22. The 2×2 coupler 21 receives a signal from alaser diode (LD) (not shown) and provides the received signal to theFFPI sensor 23 and employs a signal of the index matching unit 22 as asensor signal that will be subsequently outputted from coupler 21.

More specifically, FFPI sensor 23 receives an optical signal from the2×2 coupler 21, and uses the received signal as an input signal to indexmatching unit 22 to generate an output waveform. The index matching unit22 performs an index match for the output waveform generated by the FFPIsensor 23 and transmits transmit the matched output waveform to the 2×2coupler 21.

FIG. 3 shows the output waveform of the FFPI sensor 23 used in thepresent invention. The vertical axis represents signal intensity and thehorizontal axis represents time in the output waveform of the FFPIsensor.

When the waveform shown in FIG. 3 is affected by temperature orpressure, the vertices of the waveform move in a sideways direction. Thepresent invention has a characteristic in which such a movement of awaveform due to temperature is checked from a remote node, andtemperature is adjusted so that a waveform in an optical line terminal(OLT) coincides with the checked waveform.

FIG. 4 is a block diagram showing a construction of a WDM passiveoptical network capable of monitoring temperature according to thepresent invention. As shown, a WDM optical communication systemaccording to the present invention includes an OLT 100, remote node 200,and a plurality of optical network units (ONUs). In this exemplarysystem, OLT 100 includes 15 channels 410-1 to 410-15, a laser diode (LD)401, a coupler 402, a WGR 408, a WGR 409, a PD 407, a coupler 403, anFFPI sensor 416, an index matching unit 404, a PD 405, and processor406. The 15 channels 410-1 to 410-15 transmit data through individualwavelengths, represented as λ_(1a), λ_(2a), . . . λ_(15a) and LD 401generates an optical signal, λ₀, that is used as a reference signal fortemperature compensation as will be more fully described.

The coupler 402 divides the optical signal λ₀ into two signals andoutputs the divided signals to WGR 408 and to coupler 402. WGR 408 thenmultiplexes the reference signal λ₀, and individual wavelengths λ_(1a),λ_(2a), . . . λ_(15a) for subsequent downlink transmission to remotenode 200.

Coupler 403 receives the reference signal, λ₀, from coupler or splitter402 and sends the received signal to the FFPI sensor 416, and employs asignal of the index matching unit 404 as a sensor signal to output thesensor signal to the PD 405. More specifically, and as described withregard to FIG. 2, FFPI sensor 416 receives the signal from the coupler403 and employs the received signal as an input signal to generate anoutput waveform and the index matching unit 404 performs an indexmatching for the output waveform generated by the FFPI sensor 416 totransmit the matched output waveform to the coupler 403.

The PD 405 receives the sensor signal from coupler 403 and converts itinto an electrical signal which will be outputted to processor 406.

At the remote node 200, WGR 409 transmits wavelengths λ_(1b), λ_(2b), .. . λ_(15b) upstream to reception channels 411-1 to 411-15, and alsosends a signal, λ″₀, associated with reference signal λ₀ to PD 407. ThePD 407 converts the signal λ″₀ into an electrical signal that will beoutputted to processor 406.

Processor 406 compares the signals transmitted from PD 405 and PD 407with each other and produces necessary controls signal to equalize thetemperature of the OLT 100 with the temperature of the remote node 200as will be more fully described.

Remote node 200 includes a WGR 412, coupler 414, FFPI sensor 417, indexmatching unit 415, and WGR 413. WGR 412 demultiplexes the WDM signalreceived from the OLT 100 into individual wavelengths and provides thedemulitplexed signals to ONUs 300-1 to 300-15. The temperaturecompensation reference signal, referred to at the remote terminal asλ′₀, to coupler 414. The coupler 414 receives the λ′₀ signal from theWGR 412 and transmits the received signal to FFPI sensor 417 and employsa signal of index matching unit 415 as a sensor signal which will beoutputted. The FFPI sensor 417 receives the signal from the coupler 414and employs the received signal as an input signal to generate an outputwaveform. The index matching unit 415 performs an index matching for theoutput waveform generated by the FFPI sensor 417 and returns the matchedoutput waveform, referred to as λ″₀ to the coupler 414.

WGR 413 receives upstream signals transmitted from the ONUs 300-1 to300-15 and the sensor signal λ″₀ outputted from the coupler 414 andmultiplexes and transmits the multiplexed WDM signal OLT 100.

Processor 406, analyzes and compares the waveforms of the signals fromthe PD 405 and PD 407 and adjusts the temperature of the OLT, tosubstantially equalize the values processed. Hence, when it isdetermined that the two waveforms are not substantially equal, it isdeemed that temperature of the OLT 100 is not equalized to that of theremote node 200 and the processor 406 sends signals to thermo electriccontrollers (not shown) to adjust the temperature of the OLT 100. Hence,the temperature of the WGRs 408 and 409 are adjusted to have the sametemperature as that of the WGRs 412 and 413 in the remote node 200 underthe control of the processor 406.

While the invention has been shown and described with reference tocertain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A WDM optical communication system capable of monitoring temperaturecomprising: an optical line terminal including a laser diode forgenerating an optical signal for temperature monitoring; a first fiberFabry-Perot interferometer sensor unit for generating a first sensorsignal according to temperature by means of the optical signal; and aprocessor for monitoring temperature by means of the first sensor signaland a second sensor signal and equalizing waveforms of the two sensorsignals with each other, wherein the second sensor signal is providedexternal to the optical line terminal.
 2. The system as claimed in claim1, further comprising: a remote node comprising: a second fiberFabry-Perot interferometer sensor unit for receiving the optical signalfor temperature monitoring, generating the second sensor signalaccording to temperature, and a waveguide grating router unit fortransmitting the second sensor signal to the optical line terminal. 3.The system as claimed in claim 1, wherein the first fiber Fabry-Perotinterferometer sensor unit comprises: a 2×2 coupler; a fiber Fabry-Perotinterferometer sensor in communication with the coupler; and an indexmatching unit in communication with the interferometer and the coupler,wherein: the index matching unit performs an index matching for anoutput waveform generated by the fiber Fabry-Perot interferometer sensorassociated with the optical signal received by the coupler.
 4. Thesystem as claimed in claim 2, wherein the second fiber Fabry-Perotinterferometer sensor unit comprises: a 2×2 coupler, an fiberFabry-Perot interferometer sensor, and an index matching unit, wherein:the index matching unit performs an index matching for an outputwaveform generated by the fiber Fabry-Perot interferometer sensorassociated with the optical signal received by the coupler.
 5. Thesystem as claimed in claim 1, wherein the processor compares andanalyzes waveforms of the first sensor signal and the second sensorsignal with each other, and causes temperature adjustment in the OLT tosubstantially equalize the first sensor signal and second sensor signal.6. A method for monitoring and compensating for change in wavelength inWDM systems caused by temperature changes, the method comprising thesteps of: generating a first sensor signal associated with a transmittedreference wavelength at a transmitting site; comparing a second sensorsignal received from an external site to the first sensor signal; andadjusting the temperature at the transmitting site to substantiallyequalize the first sensor and second sensor signals.
 7. The method asclaimed in claim 6, wherein the step of generating the first sensorsignal comprises the steps of: providing the reference wavelength to aFabry-Perot sensor to generate an output waveform; and index matchingthe output waveform.
 8. The method as claimed in claim 6, wherein thestep of generating the second sensor signal comprises the steps of:providing a received reference wavelength to a Fabry-Perot sensor togenerate an output waveform; and index matching the output waveform. 9.The method as claimed in claim 6, wherein the step of adjusting thetemperature comprises the step of: providing a current to athermo-cooler at the transmitting site.
 10. The method as claimed inclaim 9, wherein the step of providing a current continues until thefirst sensor signal and the second sensor signal are substantiallyequal.